The present invention relates generally to a charged particle beam exposure system and, more particularly, to a target body position measuring method for detecting a target body position by electron beam scan to allow for fine alignment between an electronic optical system and a target body such as a semiconductor wafer in an electron beam exposure system for forming an ultrafine pattern in the order of submicrons on the target body by electron beams.
Along with the development of techniques for very large scale integrated circuits (VLSIs), a high-speed, high-resolution electron beam exposure system for forming an ultrafine pattern on a target body such as a monocrystalline silicon wafer in the order of submicrons has been steadily increasing in importance.
In order to align an electronic optical system and a wafer or target body placed on a workpiece table in a conventional electron beam exposure apparatus, an alignment or chip alignment mark on the wafer is scanned with an electron beam, and the wafer position is measured in accordance with mark detection data. When a positional error between the optical system of the electron beam exposure system and the wafer is detected, both the wafer position and electron beam position are corrected in accordance with the wafer position data, thereby performing fine alignment. However, in the conventional alignment method, when the electron beam irradiates the wafer and scans its surface to detect the alignment mark, the electron beam tends to irradiate a circuit formation area or chip area (also referred to as a chip field area) damaging the circuit pattern in the chip area. When the wafer is mechanically loaded onto the workpiece table by a chuck or holder, mechanical alignment precision is relatively low. Therefore, the size of such a mark must be increased in order to readily detect the mark with the electron beam within a short period of time. As a result, an enlarged mark itself comes close to the field area on the wafer. When the electron beam irradiates a wafer which is aligned roughly with respect to the electron beam scan in order to detect the enlarged mark, the electron beam tends to undesirably irradiate the circuit formation area, thereby damaging the circuit.
In order to solve the above drawback, the size of an alignment mark can be decreased to a width of a non-circuit area such as the small area within a dicing line area, so that the electron beam irradiates only the dicing line area. In order to effectively achieve the above technique, however, mechanical precision of wafer alignment with respect to the workpiece table must be vastly improved (e.g., the alignment error must fall within the range of .+-.20 .mu.m). Otherwise, the mark cannot be rapidly and effectively searched by electron beam irradiation of the wafer in a small area. However, it is very difficult for known mechanical alignment mechanisms to achieve extremely high precision wafer alignment. Even if such high precision can be achieved, the precision mechanism becomes complex, resulting in high cost.